Internet Engineering Task Force (IETF) A. Popov, Ed.
Request for Comments: 8471 M. Nystroem
Category: Standards Track Microsoft Corp.
ISSN: 2070-1721 D. Balfanz
Google Inc.
J. Hodges
Kings Mountain Systems
October 2018
The Token Binding Protocol Version 1.0
Abstract
This document specifies version 1.0 of the Token Binding protocol.
The Token Binding protocol allows client/server applications to
create long-lived, uniquely identifiable TLS bindings spanning
multiple TLS sessions and connections. Applications are then enabled
to cryptographically bind security tokens to the TLS layer,
preventing token export and replay attacks. To protect privacy, the
Token Binding identifiers are only conveyed over TLS and can be reset
by the user at any time.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8471.
Popov, et al. Standards Track [Page 1]

RFC 8471 The Token Binding Protocol Version 1.0 October 20181. Introduction
Servers often generate various security tokens (e.g., HTTP cookies,
OAuth tokens [RFC6749]) for applications to present when accessing
protected resources. In general, any party in possession of bearer
security tokens gains access to certain protected resource(s).
Attackers take advantage of this by exporting bearer tokens from a
user's application connections or machines, presenting them to
application servers, and impersonating authenticated users. The idea
of Token Binding is to prevent such attacks by cryptographically
binding application security tokens to the underlying TLS layer
[RFC5246]. (Note: This document deals with TLS 1.2 and therefore
refers to RFC 5246 (which has been obsoleted by RFC 8446);
[TOKENBIND-TLS13] addresses Token Binding in TLS 1.3.)
A Token Binding is established by a User Agent generating a
private-public key pair (possibly within a secure hardware module,
such as a Trusted Platform Module) per target server, providing the
public key to the server, and proving possession of the corresponding
private key, on every TLS connection to the server. The proof of
possession involves signing the Exported Keying Material (EKM)
[RFC5705] from the TLS connection with the private key. The
corresponding public key is included in the Token Binding identifier
structure (described in Section 3.2 ("TokenBinding.tokenbindingid")).
Token Bindings are long-lived, i.e., they encompass multiple TLS
connections and TLS sessions between a given client and server. To
protect privacy, Token Binding IDs are never conveyed over insecure
connections and can be reset by the user at any time, e.g., when
clearing browser cookies.
When issuing a security token to a client that supports Token
Binding, a server includes the client's Token Binding ID (or its
cryptographic hash) in the token. Later on, when a client presents a
security token containing a Token Binding ID, the server verifies
that the ID in the token matches the ID of the Token Binding
established with the client. In the case of a mismatch, the server
rejects the token (details are application specific).
In order to successfully export and replay a bound security token, an
attacker needs to also be able to use the client's private key; this
is hard to do if the key is specially protected, e.g., generated in a
secure hardware module.
Popov, et al. Standards Track [Page 3]

RFC 8471 The Token Binding Protocol Version 1.0 October 20181.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Token Binding Protocol Overview
In the course of a TLS handshake, a client and server can use the
Token Binding negotiation TLS extension [RFC8472] to negotiate the
Token Binding protocol version and the parameters (signature
algorithm, length) of the Token Binding key. This negotiation does
not require additional round trips.
Version 1.0 of the Token Binding protocol is represented by
TB_ProtocolVersion.major = 1 and TB_ProtocolVersion.minor = 0 in the
Token Binding negotiation TLS extension; see [RFC8472] ("Transport
Layer Security (TLS) Extension for Token Binding Protocol
Negotiation").
The Token Binding protocol consists of one message sent by the client
to the server, proving possession of one or more client-generated
asymmetric private keys. This message is not sent if the Token
Binding negotiation has been unsuccessful. The Token Binding message
is sent with the application protocol data over TLS.
A server receiving the Token Binding message verifies that the key
parameters in the message match the Token Binding parameters
negotiated (e.g., via [RFC8472]) and then validates the signatures
contained in the Token Binding message. If either of these checks
fails, the server rejects the binding, along with all associated
bound tokens. Otherwise, the Token Binding is successfully
established with the ID contained in the Token Binding message.
When a server supporting the Token Binding protocol receives a bound
token, the server compares the Token Binding ID in the token with the
Token Binding ID established with the client. If the bound token is
received on a TLS connection without a Token Binding or if the Token
Binding IDs do not match, the token is rejected.
This document defines the format of the Token Binding protocol
message, the process of establishing a Token Binding, the format of
the Token Binding ID, and the process of validating a bound token.
[RFC8472] describes the negotiation of the Token Binding protocol and
key parameters. [RFC8473] ("Token Binding over HTTP") explains how
the Token Binding message is encapsulated within HTTP/1.1 messages
Popov, et al. Standards Track [Page 4]

RFC 8471 The Token Binding Protocol Version 1.0 October 2018
[RFC7230] or HTTP/2 messages [RFC7540]. [RFC8473] also describes
Token Binding between multiple communicating parties: User Agent,
Identity Provider, and Relying Party.
3. Token Binding Protocol Message
The Token Binding message is sent by the client to prove possession
of one or more private keys held by the client. This message MUST be
sent if the client and server successfully negotiated the use of the
Token Binding protocol (e.g., via [RFC8472] or a different mechanism)
and MUST NOT be sent otherwise. This message MUST be sent in the
client's first application protocol message. This message MAY also
be sent in subsequent application protocol messages, proving
possession of additional private keys held by the same client; this
information can be used to facilitate Token Binding between more than
two communicating parties. For example, [RFC8473] specifies an
encapsulation of the Token Binding message in HTTP application
protocol messages, as well as scenarios involving more than two
communicating parties.
The Token Binding message format is defined using the TLS
presentation language (see Section 4 of [RFC5246]):
enum {
rsa2048_pkcs1.5(0), rsa2048_pss(1), ecdsap256(2), (255)
} TokenBindingKeyParameters;
struct {
opaque modulus<1..2^16-1>;
opaque publicexponent<1..2^8-1>;
} RSAPublicKey;
struct {
opaque point <1..2^8-1>;
} TB_ECPoint;
struct {
TokenBindingKeyParameters key_parameters;
uint16 key_length; /* Length (in bytes) of the following
TokenBindingID.TokenBindingPublicKey */
select (key_parameters) {
case rsa2048_pkcs1.5:
case rsa2048_pss:
RSAPublicKey rsapubkey;
case ecdsap256:
TB_ECPoint point;
} TokenBindingPublicKey;
} TokenBindingID;
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RFC 8471 The Token Binding Protocol Version 1.0 October 2018
enum {
(255) /* No initial TB_ExtensionType registrations */
} TB_ExtensionType;
struct {
TB_ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} TB_Extension;
enum {
provided_token_binding(0), referred_token_binding(1), (255)
} TokenBindingType;
struct {
TokenBindingType tokenbinding_type;
TokenBindingID tokenbindingid;
opaque signature<64..2^16-1>; /* Signature over the concatenation
of tokenbinding_type,
key_parameters, and EKM */
TB_Extension extensions<0..2^16-1>;
} TokenBinding;
struct {
TokenBinding tokenbindings<132..2^16-1>;
} TokenBindingMessage;
The Token Binding message consists of a series of TokenBinding
structures, each containing the type of the Token Binding, the
TokenBindingID, and a signature using the Token Binding key,
optionally followed by TB_Extension structures.
3.1. TokenBinding.tokenbinding_type
This document defines two Token Binding types:
o provided_token_binding - used to establish a Token Binding when
connecting to a server.
o referred_token_binding - used when requesting tokens that are
intended to be presented to a different server.
[RFC8473] describes a use case for referred_token_binding where Token
Bindings are established between multiple communicating parties:
User Agent, Identity Provider, and Relying Party. The User Agent
sends referred_token_binding to the Identity Provider in order to
prove possession of the Token Binding key it uses with the Relying
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RFC 8471 The Token Binding Protocol Version 1.0 October 2018
Party. The Identity Provider can then bind the token it is supplying
(for presentation to the Relying Party) to the Token Binding ID
contained in referred_token_binding.
An implementation MUST ignore any unknown Token Binding types.
3.2. TokenBinding.tokenbindingid
The ID of the Token Binding established as a result of Token Binding
message processing contains the identifier of the negotiated key
parameters, the length (in bytes) of the Token Binding public key,
and the Token Binding public key itself. The Token Binding ID can be
obtained from the TokenBinding structure by discarding the Token
Binding type, signature, and extensions.
When rsa2048_pkcs1.5 or rsa2048_pss is used, RSAPublicKey.modulus and
RSAPublicKey.publicexponent contain the modulus and exponent of a
2048-bit RSA public key represented in big-endian format, with
leading zero bytes omitted.
When ecdsap256 is used, TB_ECPoint.point contains the X coordinate
followed by the Y coordinate of a Curve P-256 key. The X and Y
coordinates are unsigned 32-byte integers encoded in big-endian
format, preserving any leading zero bytes. Future specifications may
define Token Binding keys using other elliptic curves with their
corresponding signature and point formats.
Token Binding protocol implementations SHOULD make Token Binding IDs
available to the application as opaque byte sequences, so that
applications do not rely on a particular Token Binding ID structure.
For example, server applications will use Token Binding IDs when
generating and verifying bound tokens.
3.3. TokenBinding.signature
When rsa2048_pkcs1.5 is used, TokenBinding.signature contains the
signature generated using the RSASSA-PKCS1-v1_5 signature scheme
defined in [RFC8017] with SHA256 [FIPS.180-4.2015] as the hash
function.
When rsa2048_pss is used, TokenBinding.signature contains the
signature generated using the RSA Probabilistic Signature Scheme
(RSASSA-PSS) defined in [RFC8017] with SHA256 as the hash function.
MGF1 with SHA256 MUST be used as the mask generation function (MGF),
and the salt length MUST equal 32 bytes.
When ecdsap256 is used, TokenBinding.signature contains a pair of
32-byte integers, R followed by S, generated with the Elliptic Curve
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RFC 8471 The Token Binding Protocol Version 1.0 October 2018
Digital Signature Algorithm (ECDSA) using Curve P-256 and SHA256 as
defined in [FIPS.186-4.2013] and [ANSI.X9-62.2005]. R and S are
encoded in big-endian format, preserving any leading zero bytes.
The signature is computed over the byte string representing the
concatenation of:
o The TokenBindingType value contained in the
TokenBinding.tokenbinding_type field,
o The TokenBindingKeyParameters value contained in the
TokenBindingID.key_parameters field, and
o The EKM value obtained from the current TLS connection.
Please note that TLS 1.2 and earlier versions support renegotiation,
which produces a new TLS master secret for the same connection, with
the associated session keys and EKM value. TokenBinding.signature
MUST be a signature of the EKM value derived from the TLS master
secret that produced the session keys encrypting the TLS
application_data record(s) containing this TokenBinding. Such use of
the current EKM for the TLS connection makes replay of bound tokens
within renegotiated TLS sessions detectable but requires the
application to synchronize Token Binding message generation and
verification with the TLS handshake state.
Specifications defining the use of Token Binding with application
protocols, such as Token Binding over HTTP [RFC8473], MAY prohibit
the use of TLS renegotiation in combination with Token Binding,
obviating the need for such synchronization. Alternatively, such
specifications need to define (1) a way to determine which EKM value
corresponds to a given TokenBindingMessage and (2) a mechanism that
prevents a TokenBindingMessage from being split across TLS
renegotiation boundaries due to TLS message fragmentation; see
Section 6.2.1 of [RFC5246]. Note that application-layer messages
conveying a TokenBindingMessage may cross renegotiation boundaries in
ways that make processing difficult.
The EKM is obtained using the keying material exporters for TLS as
defined in [RFC5705], by supplying the following input values:
o Label: The ASCII string "EXPORTER-Token-Binding" with no
terminating NUL.
o Context value: No application context supplied.
o Length: 32 bytes.
Popov, et al. Standards Track [Page 8]

RFC 8471 The Token Binding Protocol Version 1.0 October 20183.4. TokenBinding.extensions
A Token Binding message may optionally contain a series of
TB_Extension structures, each consisting of an extension_type and
extension_data. The structure and meaning of extension_data depends
on the specific extension_type.
Initially, no extension types are defined (see Section 6.3
("Token Binding Extensions Registry")). One of the possible uses of
extensions envisioned at the time of this writing is attestation:
cryptographic proof that allows the server to verify that the Token
Binding key is hardware bound. The definitions of such Token Binding
protocol extensions are outside the scope of this specification.
4. Establishing a Token Binding4.1. Client Processing Rules
The client MUST include at least one TokenBinding structure in the
Token Binding message. When a provided_token_binding is included,
the key parameters used in a provided_token_binding MUST match those
negotiated with the server (e.g., via [RFC8472] or a different
mechanism).
The client MUST generate and store Token Binding keys in a secure
manner that prevents key export. In order to prevent cooperating
servers from linking user identities, the scope of the Token Binding
keys MUST NOT be broader than the scope of the tokens, as defined by
the application protocol.
When the client needs to send a referred_token_binding to the
Identity Provider, the client SHALL construct the referred
TokenBinding structure in the following manner:
o Set TokenBinding.tokenbinding_type to referred_token_binding.
o Set TokenBinding.tokenbindingid to the Token Binding ID used with
the Relying Party.
o Generate TokenBinding.signature, using the EKM value of the TLS
connection to the Identity Provider, the Token Binding key
established with the Relying Party, and the signature algorithm
indicated by the associated key parameters. Note that these key
parameters may differ from the key parameters negotiated with the
Identity Provider.
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RFC 8471 The Token Binding Protocol Version 1.0 October 2018
Conveying referred Token Bindings in this fashion allows the Identity
Provider to verify that the client controls the Token Binding key
used with the Relying Party.
4.2. Server Processing Rules
The triple handshake vulnerability in TLS 1.2 and older TLS versions
affects the security of the Token Binding protocol, as described in
Section 7 ("Security Considerations"). Therefore, the server
MUST NOT negotiate the use of the Token Binding protocol with these
TLS versions, unless the server also negotiates the extended master
secret TLS extension [RFC7627] and the renegotiation indication TLS
extension [RFC5746].
If the use of the Token Binding protocol was not negotiated but the
client sends a Token Binding message, the server MUST reject any
contained bindings.
If the Token Binding type is "provided_token_binding", the server
MUST verify that the signature algorithm (including an elliptic curve
in the case of ECDSA) and key length in the Token Binding message
match those negotiated with this client (e.g., via [RFC8472] or a
different mechanism). In the case of a mismatch, the server MUST
reject the binding. Token Bindings of type "referred_token_binding"
may use different key parameters than those negotiated with this
client.
If the Token Binding message does not contain at least one
TokenBinding structure or if a signature contained in any
TokenBinding structure is invalid, the server MUST reject the
binding.
Servers MUST ignore any unknown extensions. Initially, no extension
types are defined (see Section 6.3 ("Token Binding Extensions
Registry")).
If all checks defined above have passed successfully, the Token
Binding between this client and server is established. The Token
Binding ID(s) conveyed in the Token Binding message can be provided
to the server-side application. The application may then use the
Token Binding IDs for bound security token creation and validation;
see Section 5.
If a Token Binding is rejected, any associated bound tokens presented
on the current TLS connection MUST also be rejected by the server.
The effect of this is application specific, e.g., failing requests, a
requirement for the client to re-authenticate and present a different
token, or connection termination.
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RFC 8471 The Token Binding Protocol Version 1.0 October 20185. Bound Security Token Creation and Validation
Security tokens can be bound to the TLS layer in a variety of ways,
e.g., by embedding the Token Binding ID or its cryptographic hash in
the token or by maintaining a database mapping tokens to Token
Binding IDs. The specific method of generating bound security tokens
is defined by the application and is beyond the scope of this
document. Note that applicable security considerations are outlined
in Section 7.
Either or both clients and servers MAY create bound security tokens.
For example, HTTPS servers employing Token Binding for securing their
HTTP cookies will bind these cookies. In the case of a server-
initiated challenge-response protocol employing Token Binding and
TLS, the client can, for example, incorporate the Token Binding ID
within the signed object it returns, thus binding the object.
Upon receipt of a security token, the server attempts to retrieve
Token Binding ID information from the token and from the TLS
connection with the client. Application-provided policy determines
whether to honor non-bound (bearer) tokens. If the token is bound
and a Token Binding has not been established for the client
connection, the server MUST reject the token. If the Token Binding
ID for the token does not match the Token Binding ID established for
the client connection, the server MUST reject the token.
6. IANA Considerations
This section establishes a new IANA registry titled "Token Binding
Protocol" with subregistries "Token Binding Key Parameters", "Token
Binding Types", and "Token Binding Extensions". It also registers a
new TLS exporter label in the "TLS Exporter Labels" registry.
6.1. Token Binding Key Parameters Registry
This document establishes a subregistry for identifiers of Token
Binding key parameters titled "Token Binding Key Parameters" under
the "Token Binding Protocol" registry.
Entries in this registry require the following fields:
o Value: The octet value that identifies a set of Token Binding key
parameters (0-255).
o Description: The description of the Token Binding key parameters.
o Reference: A reference to a specification that defines the Token
Binding key parameters.
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RFC 8471 The Token Binding Protocol Version 1.0 October 2018
This registry operates under the "Specification Required" policy as
defined in [RFC8126]. The designated expert will require the
inclusion of a reference to a permanent and readily available
specification that enables the creation of interoperable
implementations using the identified set of Token Binding key
parameters.
An initial set of registrations for this registry follows:
Value: 0
Description: rsa2048_pkcs1.5
Specification: This document
Value: 1
Description: rsa2048_pss
Specification: This document
Value: 2
Description: ecdsap256
Specification: This document
6.2. Token Binding Types Registry
This document establishes a subregistry for Token Binding type
identifiers titled "Token Binding Types" under the "Token Binding
Protocol" registry.
Entries in this registry require the following fields:
o Value: The octet value that identifies the Token Binding type
(0-255).
o Description: The description of the Token Binding type.
o Reference: A reference to a specification that defines the Token
Binding type.
This registry operates under the "Specification Required" policy as
defined in [RFC8126]. The designated expert will require the
inclusion of a reference to a permanent and readily available
specification that enables the creation of interoperable
implementations using the identified Token Binding type.
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RFC 8471 The Token Binding Protocol Version 1.0 October 2018
An initial set of registrations for this registry follows:
Value: 0
Description: provided_token_binding
Specification: This document
Value: 1
Description: referred_token_binding
Specification: This document
6.3. Token Binding Extensions Registry
This document establishes a subregistry for Token Binding extensions
titled "Token Binding Extensions" under the "Token Binding Protocol"
registry.
Entries in this registry require the following fields:
o Value: The octet value that identifies the Token Binding extension
(0-255).
o Description: The description of the Token Binding extension.
o Reference: A reference to a specification that defines the Token
Binding extension.
This registry operates under the "Specification Required" policy as
defined in [RFC8126]. The designated expert will require the
inclusion of a reference to a permanent and readily available
specification that enables the creation of interoperable
implementations using the identified Token Binding extension. This
document creates no initial registrations in the "Token Binding
Extensions" registry.
6.4. Registration of Token Binding TLS Exporter Label
This document adds the following registration in the "TLS Exporter
Labels" registry:
Value: EXPORTER-Token-Binding
DTLS-OK: Y
Recommended: Y
Reference: This document
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RFC 8471 The Token Binding Protocol Version 1.0 October 20187. Security Considerations7.1. Security Token Replay
The goal of the Token Binding protocol is to prevent attackers from
exporting and replaying security tokens and from thereby
impersonating legitimate users and gaining access to protected
resources. Bound tokens can be replayed by malware present in
User Agents; this may be undetectable to a server. However, in order
to export bound tokens to other machines and successfully replay
them, attackers also need to export corresponding Token Binding
private keys. Token Binding private keys are therefore high-value
assets and SHOULD be strongly protected, ideally by generating them
in a hardware security module that prevents key export.
The manner in which a token is bound to the TLS layer is defined by
the application and is beyond the scope of this document. However,
the resulting bound token needs to be integrity-protected, so that an
attacker cannot remove the binding or substitute a Token Binding ID
of their choice without detection.
The Token Binding protocol does not prevent cooperating clients from
sharing a bound token. A client could intentionally export a bound
token with the corresponding Token Binding private key or perform
signatures using this key on behalf of another client.
7.2. Downgrade Attacks
The Token Binding protocol MUST be negotiated using a mechanism that
prevents downgrade attacks. For example, [RFC8472] specifies a TLS
extension for Token Binding negotiation. TLS detects handshake
message modification by active attackers; therefore, it is not
possible for an attacker to remove or modify the "token_binding"
extension without breaking the TLS handshake. The signature
algorithm and key length used in the TokenBinding of type
"provided_token_binding" MUST match the negotiated parameters.
7.3. Token Binding Key-Sharing between Applications
Existing systems provide a variety of platform-specific mechanisms
for certain applications to share tokens, e.g., to enable "single
sign-on" scenarios. For these scenarios to keep working with bound
tokens, the applications that are allowed to share tokens will need
to also share Token Binding keys. Care must be taken to restrict the
sharing of Token Binding keys to the same group(s) of applications
that shares the same tokens.
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RFC 8471 The Token Binding Protocol Version 1.0 October 20187.4. Triple Handshake Vulnerability in TLS 1.2 and Older TLS Versions
The Token Binding protocol relies on the TLS exporters [RFC5705] to
associate a TLS connection with a Token Binding. The triple
handshake attack [TRIPLE-HS] is a known vulnerability in TLS 1.2 and
older TLS versions, allowing the attacker to synchronize keying
material between TLS connections. The attacker can then successfully
replay bound tokens. For this reason, the Token Binding protocol
MUST NOT be negotiated with these TLS versions, unless the extended
master secret TLS extension [RFC7627] and the renegotiation
indication TLS extension [RFC5746] have also been negotiated.
8. Privacy Considerations
The Token Binding protocol uses persistent, long-lived Token Binding
IDs. To protect privacy, Token Binding IDs are never transmitted in
clear text and can be reset by the user at any time, e.g., when
clearing browser cookies. Some applications offer a special privacy
mode where they don't store or use tokens supplied by the server,
e.g., "in private" browsing. When operating in this special privacy
mode, applications SHOULD use newly generated Token Binding keys and
delete them when exiting this mode; otherwise, they SHOULD NOT
negotiate Token Binding at all.
In order to prevent cooperating servers from linking user identities,
the scope of the Token Binding keys MUST NOT be broader than the
scope of the tokens, as defined by the application protocol.
A server can use tokens and Token Binding IDs to track clients.
Client applications that automatically limit the lifetime or scope of
tokens to maintain user privacy SHOULD apply the same validity time
and scope limits to Token Binding keys.
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